The invention relates to a method for controlling a high pressure gas injection internal combustion engine. The invention also relates to a fuel system for a high pressure gas injection internal combustion engine.
The invention can be applied in heavy-duty vehicles such as trucks, buses and construction equipment, e.g. working machines. The invention can also be applied to cars. Although the invention will be described with respect to a truck, the invention is not restricted to this particular vehicle type.
In internal combustion engines with high pressure gas injection (HPGI), there are usually requirements to dispense of high pressure gas, e.g. during a rapid pressure decrease in the injection system due to a decrease in the engine load, or during an engine stoppage. A need to vent boil-off gas from the gaseous fuel storage is another common reason for disposing of gas. Such disposal will of course create an environmental disturbance, since it involves emitting unburned hydrocarbons into the atmosphere.
CA2868338A1 suggests, for an internal combustion engine with direct gas injection, capturing vented gaseous fuel, storing it in an accumulator, and reintroducing it for engine combustion at a later time. During high load operations of engine, the gaseous fuel from the accumulator is introduced upstream, of an air intake compressor and a gas and air premix is thereby added in the cycles in the cylinders before the regular direct gas injection. A problem with this solution is that it gives little possibilities to control the process of burning the captured fuel. Such control is desirable in a vehicle, in which the engine undergo a large number of load changes during a relatively short span of time.
It is desirable to reduce emissions from fuel systems of high pressure gay injection internal combustion engines. It is also desirable to reduce waste gas emissions from high pressure gas injection internal combustion engines. It is also desirable to improve the control over fuel system emission reduction measures in high pressure gas injection internal combustion engines.
According to an aspect of the invention, a method is provided for controlling a high pressure gas injection internal combustion engine comprising at least one cylinder, comprising supplying a first gaseous fuel from a first gas injection system and injecting the first gaseous fuel in at least one of said at least one cylinder, characterized by
converting waste gas from the first gas injection system to a second fuel, and
injecting the second fuel obtained by the conversion into at least one of said at least one cylinder.
Differing from said CA2868338A1, the invention provides for a large degree of control when using the waste gas in engine operations. In CA2868338A1 the gaseous fuel from the accumulator is provided upstream of the compressor, and will therefore be distributed in a fuel and air premix to all cylinders, which does not give very much room for control over the combustion of the gaseous fuel from the accumulator. Differing from this, since the invention provides for the waste gas from the first gas injection system to be converted to a second fuel, and injected into at least one of said at least one cylinder, the amount and timing of the combustion of the second fuel will be highly controllable, e.g. in view of the operational condition of the engine. The improved control provided by the invention is particularly useful in a vehicle, in which the engine undergoes a large number of load changes during a relatively short span of time, and fast adaptions to new load situations is highly beneficial.
Also, the invention also provides for running a dual fuel HPGI vehicle with a single fuel. More particularly, the invention makes it possible, as exemplified below, to convert the waste gas from the first gaseous fuel, e.g. in the form of liquid natural gas (LNG), to a second fuel in the form of pilot fuel or igniter fuel, e.g. dimethyl ether (DME), for pilot injections in the cycles in the engine cylinders. Thereby, the invention provides the dual benefit of eliminating waste gas emission from a HPGI vehicle and eliminating the need for adapting the vehicle to be fuelled with two fuel types, thereby reducing the complexity and cost of the vehicle.
The step of injecting the second fuel preferably comprises injecting the second fuel obtained by the conversion into the at least one cylinder into which the first gaseous fuel is injected. The step of injecting the first gaseous fuel may comprise injecting the first gaseous fuel in a combustion cycle in the cylinder, and the step of injecting the second fuel may comprise injecting the second fuel obtained by the conversion in a further injection in said combustion cycle. The injection of the first gaseous fuel is a main injection, and the injection of the second fuel is a pilot injection. The pilot inject c it is adapted to ignite the combustion of the main injection. As suggested, thereby, besides providing a high degree of control over the emission reducing waste gas removal process, a vehicle provided with a HPGI engine needs to be equipped for filling of one fuel type only.
The invention advantageously provides for emission reduction of waste gas obtained in different manners. For example, where the first gas injection system comprises a first container, e.g. an LNG tank, the step of converting waste gas from the first gas injection system to the second fuel may comprise converting boil-off gas from the first container to the second fuel. Where the first gas injection system comprises at least one first injector for the injection of the first gaseous fuel, and a first conduit for guiding the first gaseous fuel, e.g. from a first container and a high pressure pump, to the at least one of said at least one cylinder, the step of converting waste gas from the first gas injection system to the second fuel may comprise converting first gaseous fuel received from the first conduit to the second fuel.
Preferably the method comprises storing the waste gas from the first gas injection system in a second container before converting the waste gas to the second fuel. Thereby, the conversion process will be less dependent on variation in the production of waste gas, since said storage may provide a buffer function.
Preferably, the method comprises storing the second fuel obtained by the conversion before injecting the second fuel. Thereby, the control over the use of the second fuel is improved, since it is not dependent on the conversion production rate, but can be adapted to requirements like engine load etc.
Preferably, the second fuel obtained by the conversion is a liquid fuel, for example DME. The conversion to the second fuel may comprise reforming the waste gas to syngas, e.g. in a steam reformer as exemplified below, involving mixing the waste gas with water. Methanol may be produced based on the syngas, and DME may be produced based on the methanol, e.g. in a DME reactor. This provides a beneficial conversion method for the implementation of the invention. Water and methanol remaining after the DME reactor may be separated and fed into the steam reformer.
Preferably, the method comprises using at least one surplus product, such as hydrogen, unreacted carbon monoxide and/or rest methane, from the step of producing methanol for heat generation for the step of reforming the waste gas to syngas. Thereby bi-products obtained in the conversion is used for fuelling the conversion process itself which is cost effective and reduces emissions from the fuel system further. As also exemplified below, the heat generation may be provided e.g. by means of a burner or a catalyst.
Advantageously, where a fuel converter for the conversion of the waste gas to the second fuel is provided, the method comprises guiding excess gas from the fuel converter to the engine. This provides an advantageous way of disposing of excess gases in an environmentally beneficial manner, e.g. where the fuel converter comprises a steam reformer for which heat is generated as exemplified above, and where the heat generation produces the excess gases. The excess gases may be mixed in an air intake system with intake air for the engine or they may be directly injected into one or more of the cylinders, to be converted to carbon dioxide and water in a combustion. In alternative embodiments, the fuel system may be arranged to guide excess gas to an exhaust after treatment system for the engine.
In some embodiments, the conversion to the second fuel comprises mixing the waste gas with air as a reactant for an autotherm reformer for producing dimethyl ether (DME). Thus, such autotherm reformer may be provided as an alternative to said steam reformer, for providing syngas, and a reactor may be provided for a direct conversion from syngas to DME.
In further embodiments, the conversion to the second fuel may comprise a use of a Fisher-Tropsch reactor to produce the second fuel in the form of a hydrocarbon. Thus, such embodiments may provide an alternative to DME production, providing a diesel like hydrocarbon.
According to another aspect of the invention, a fuel system for a high pressure gas injection internal combustion engine is provided comprising at least one cylinder, comprising
a first gas injection system for injecting a first gaseous fuel into at least one of the at least one cylinder,
a second fuel injection system arranged to receive waste gas from the first gas injection system,
characterised in that the second fuel injection system comprises a fuel converter for converting the received waste gas to a second fuel, and in that the second fuel injection system is arranged to inject the second fuel into at least one of the at least one cylinder.
Advantages provided by the fuel system is understood from the description of embodiments of the method above. The first gas injection system may comprise a main injector for each cylinder of the engine, and the second fuel injection system may comprise a pilot injector in each cylinder of the engine.
Preferably, the fuel converter comprises a steam reformer for reforming the waste gas to syngas. A methanol reactor may be provided for producing methanol based on the syngas from the steam reformer, and a DME reactor may be provided for producing the DME based on the methanol. The fuel converter may comprises a heat generating device arranged to receive at least one surplus product from the methanol reactor and to generate heat for the steam reformer by means of the surplus product. Since preparing DME based on methane requires oxygen, the use of a steam reformer involves an advantageous manner of introducing the oxygen with water. It is advantageous since it, as opposed to partial oxidation, i.e. introducing the oxygen with air, does not introduce nitrogen which may reduce the purity of the DME and produce excessive heat.
In alternative embodiments, the fuel converter comprises an autotherm reformer for producing the second fuel in the form of DME. An advantage with the use of an authotherm reformer is that it does not require any external heat for its process.
Further advantages and advantageous futures of the invention are disclosed in the following description and in the dependent claims.